In recent years, development of photon counting CT (Computed Tomography) devices has been carried forward, equipped with a detector that employs a photon counting mode (a photon counting detector). Unlike a detector of charge integration type, which is employed in conventional CT devices, the photon counting detector is capable of counting x-ray photons individually, which have been entered into detector elements. This configuration allows measurement of energy of the entered x-ray photons on a photon-by-photon basis, featuring that much more information can be obtained when compared to the conventional CT devices.
The detector element in the photon counting detector is provided with a semiconductor layer made of cadmium zinc telluride (CZT), cadmium telluride (CdTe), or the like, and outputs a pulse signal every time the x-ray photon is entered, in response to thus entered x-ray photon.
However, time is required for the detector element to generate the pulse signal and for a measuring circuit to repose, which measures the pulse signal outputted from the detector element. Therefore, it is known that if X-rays are incident at a high rate, a subsequent x-ray photon is prone to be entered, prior to attenuation of the pulse signal of the x-ray photon previously entered, resulting in that the pulse signals are piled up. This phenomenon is referred to as “pile-up”, and due to this phenomenon, the photon counting detector may fail to measure correctly the number of x-ray photons and its energy. To address this problem, there is known a photon counting detector comprising a plurality of detector elements arranged in a region corresponding to one pixel, where X-rays are divided in a planar fashion and detected respectively, thereby adjusting the amount of the x-ray photons entered into one detector element, and reducing such generation of pile-up.
In a conventional CT device, a charge integration detector is employed, and a method has fairly matured, where an image is generated from measured data (current data) acquired by this charge integration detector, and a diagnosis is performed by using thus generated image. Therefore, it is also desired in a photon counting CT device, to acquire data of current simultaneously, in addition to an output of counting x-ray photons. In order to achieve this object, there is disclosed in the patent document 1, a circuit configuration where detector elements using CdTe are connected to both a photon measuring circuit and a current measuring circuit.